JPH0525580B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing large diameter steel pipes with excellent low-temperature toughness in the heat affected zone (HAZ), from low heat input welding to high heat input welding. [In the steel industry, it is most desirable to apply to UOE, spiral, and cage steel pipes. Large-diameter steel pipes manufactured by this manufacturing method can be used as large-diameter line pipes for transporting natural gas and crude oil. ] (Conventional technology) The HAZ toughness of low alloy steel is determined by (1) grain size;
(2) various metallurgical factors such as the dispersion state of hardened phases such as high carbon island martensite (M) and upper bainite (Bu), (3) presence or absence of grain boundary embrittlement, and (4) micro-segregation of elements. ruled by. In particular, it is known that the grain size of HAZ has a large effect on low-temperature toughness.
A number of techniques for making microstructures smaller have been developed and put into practical use. According to Japanese Patent Application Laid-open No. 61-79745, steel in which Ti oxide (mainly Ti ₂ O ₃ ) is finely dispersed can have a small HAZ structure even near the melting line, and has excellent low-temperature toughness. is disclosed. In addition, as a method for finely dispersing Ti oxide, Japanese Patent Application Laid-Open No. 61-229461 describes a cooling rate of 20% during solidification.
Casting at ~400°C/min is disclosed.
However, when a slab is actually manufactured by a continuous casting method, it is very difficult to control the cooling rate over the entire slab including the width direction and thickness direction. In the center of the slab, where the cooling rate is slow, Ti oxide tends to become coarser and the number of Ti oxides decreases significantly, so the HAZ structure is not refined enough.
Deterioration of HAZ toughness is unavoidable. (Problems to be Solved by the Invention) The present invention proposes a manufacturing method for inexpensively manufacturing large-diameter steel pipes with excellent low-temperature toughness. including
The structure is refined throughout the HAZ and has excellent low-temperature toughness. Means for Solving the Problems The gist of the present invention is that C: 0.005 to 0.15%, Si: 0.5
% or less, Mn: 0.5 to 2.2%, P: 0.02% or less, S:
0.005% or less, Al: 0.005% or less, Nb: 0.01~0.1
%, Ti: 0.005-0.03%, N: 0.001-0.005%,
The basic component is O: 0.0015-0.006%, V: 0.01-0.1%, Ni: 0.05-1.0%, Cu:
0.05~1.0%, Cr: 0.05~0.5%, Mo: 0.05~0.4
%, B: 0.0005-0.003%, Ca: 0.001-0.005%,
REM: When manufacturing slabs by continuous casting method from molten steel containing 0.01 to 0.05% of one or more types, with the remainder consisting of iron and unavoidable impurities,
The secondary cooling water on the short side of the slab has a water volume density of 350 to 840/
m ² and 50 to 500 Ti-containing oxides/mm ²
This is a method for producing large diameter steel pipes with excellent low-temperature toughness, which is characterized by producing slabs containing the same, followed by heating and rolling, and then butt-welding the edges of the steel plates. The present invention will be explained below. Ti oxide (mainly Ti ₂ O ₃ ) has a small ability to suppress the coarsening of γ grains, but during γ-α transformation, Ti ₂ O ₃ present in γ grains is the main component, and it produces radially fine acyl ferrite. (AF) is generated and significantly refines the HAZ structure. Ti ₂ O ₃ is stable even in the region heated to 1400°C or higher near the melting line (coarse grain region), and is effective in refining the structure even in this region. Also Ti, O, N
If the balance is appropriate, fine TiN will also be generated,
This is an area heated to below 1350℃ (sub-coarse area)
This suppresses the coarsening of γ grains and refines the HAZ structure. As a result, the HAZ structure becomes finer over the entire area, resulting in extremely excellent low-temperature toughness. In order to finely disperse Ti ₂ O ₃ and TiN in steel, it is necessary to optimize the amounts of Ti, O, and N, and to maintain the liquidus line to solidus line when producing slabs from molten steel by continuous casting. It is necessary to optimize the cooling rate between. This point will be explained below. First, the amounts of Ti, O, and N are determined from Ti: 0.005 to 0.005, respectively.
0.03%, O: 0.0015-0.006%, N: 0.001-0.005
%. The lower limit of the amount of Ti, O, and N is Ti ₂ O ₃ ,
This is the minimum amount required to generate TiN. The upper limit of Ti is set to prevent deterioration of low-temperature toughness due to the formation of TiC, and the upper limit of O is set to prevent deterioration of the cleanliness and toughness of the steel due to the formation of nonmetallic inclusions. Further, the upper limit of the N content was set to 0.005% in order to prevent deterioration of HAZ toughness due to solid solution N. However, it is not possible to stably obtain both fine precipitates of Ti ₂ O ₃ and TiN at the same time by simply limiting the amount of each element, so the balance between the amounts of Ti, O, and N must be adjusted to -0.010%≦[Ti] -2[O]-3.4[N]≦+0.01
Five
% is desirable. Next, in order to finely disperse Ti ₂ O ₃ in steel, it is important to specify and control the solidification cooling rate when producing slabs from molten steel by continuous casting. It is very difficult in practice to define and control the rate of solidification cooling throughout the piece. Furthermore, when manufacturing large-diameter steel pipes, it is not necessarily necessary to specify the solidification and cooling rate of the entire slab, considering that steel plate edges are usually butt welded. As a result of research conducted by the present inventors regarding this point, by optimizing the amount of water for secondary cooling of the short side of the slab during continuous casting, the cooling rate during solidification of only the edge of the short side of the slab could be reduced. We have invented a new method for manufacturing large-diameter steel pipes that can be controlled and then butt-welded at the edges. For secondary cooling on the short side of the slab, set the water density to 350 to 840.
/ ^m2 , it is possible to contain 50 to 500 Ti-containing oxides/ ^mm2 . As a result, extremely excellent HAZ toughness can be obtained when the steel plate edges are butt welded. The water density for secondary cooling on the short side of the slab should be set to 350 to 840.
/m ² is because the water density of secondary cooling is
This is because if it is less than 350/m ² , the number of Ti oxides on the short side of the slab will decrease, and the HAZ toughness will deteriorate.
Also, if the water density for secondary cooling is 840/m ² or more, surface defects will occur in the slab. After rolling the slab, when butt welding the edges of the steel plates, the maximum amount for development processing corresponds to the thickness of the short side of the slab of 50 mm, so the secondary cooling zone on the short side of the slab has a solidification thickness of 50 mm. A range within this range is sufficient. The number of Ti-containing oxides in the present invention refers to Ti-containing oxides with a size of 0.1 μm or more observed under a 1000x optical microscope. 50% fine Ti oxide on the edge of the slab
After dispersing ~500 pieces/ ^mm2 , these slabs are heated and rolled, and the edges of the steel plates are butt welded to produce large diameter steel pipes.The HAZ structure is refined even near the melting line, and it has excellent low-temperature toughness. have Further, even if Ti ₂ O ₃ and TiN are finely dispersed in the steel, excellent HAZ toughness cannot be obtained unless the basic components are appropriate. This point will be explained below. The lower limit of 0.005% of C is the minimum amount to ensure the strength of the base metal and welded part and to exhibit these effects when adding Nb, V, etc. But C
If the amount is too large, it not only adversely affects the low-temperature toughness of the base metal and welded joint, but also deteriorates weldability and HAZ toughness, so the upper limit was limited to 0.15%. Si is an element contained in deoxidized steel, but since it is an element that deteriorates weldability and HAZ toughness, the upper limit was set at 0.5%. Mn is an essential element for ensuring strength and toughness, and its lower limit is 0.6%. However, too much Mn increases the hardenability of the steel and deteriorates weldability and HAZ toughness, so the upper limit was set at 2.5%. The reason why the impurities P and S in the steel of the present invention are set to 0.02% or less and 0.005% or less, respectively, is that the base material,
This is to further improve the low temperature toughness of the HAZ. A reduction in the amount of P tends to reduce the tendency for grain boundary fracture at the joint, and a reduction in the amount of S tends to suppress the formation of grain boundary ferrite. The most preferable amounts of P and S are 0.01% and 0.0020% or less, respectively. Al is an element generally included in deoxidized steel,
In the present invention, it is an undesirable element, and its upper limit is
It was set as 0.005%. This is because when Al is contained in steel, it combines with O to form Ti ₂ O ₃ . Deoxidation is possible with Ti alone, and in the present invention, Al
The smaller the amount, the better, preferably 0.002% or less. Nb is an important essential element in the steel of the present invention,
In high-strength steel, it is difficult to obtain excellent joint toughness without adding Nb. Nb has the function of suppressing ferrite generated at the γ grain boundaries and promoting the generation of fine AF mainly composed of Ti ₂ O ₃ . A minimum of 0.01% Nb is required to obtain this effect.
Quantity is required. However, if the amount of Nb is too large, it will hinder the formation of fine AF, so the upper limit was set at 0.1%. Next, V, Ni, Cu, Cr, Mo, B, Ca, REM
The reason for adding is explained below. The main purpose of adding these elements to the basic components is to improve the characteristics of the steel of the present invention without impairing its characteristics.
This is to improve properties such as strength and toughness.
Therefore, the amount added must be limited. V is an element that has almost the same effect as Nb, but
Below 0.01%, the effect is small, and an upper limit of 0.06% is acceptable. Ni improves the strength and toughness of the base metal without adversely affecting weldability and joint toughness, but
If it exceeds 1.0%, it is unfavorable for weldability, so the upper limit was set at 2.0%. Cu has almost the same effects as Ni, as well as corrosion resistance and hydrogen-induced cracking resistance, but if it exceeds 1.0%, Cu cracks will occur during hot rolling.
It becomes difficult to manufacture. For this reason, the upper limit was set at 1.0%. Cr increases the strength of the base metal and weld zone, but too much Cr deteriorates weldability and joint toughness. The upper limit is 0.5%. Mo is an element that improves both the strength and toughness of the base metal, but if it is present in too much, it is undesirable as it causes deterioration of the toughness and weldability of the base metal and joints, similar to Cr. Its upper limit is 0.4%. Note that the lower limit of the amount of Ni, Cu, Cr, and Mo added should be the minimum value for obtaining the effect on the material, and each is 0.05%. B is an element that increases the hardenability and strength of steel. The solid solution B segregated at the γ grain boundaries of the joint suppresses the formation of ferrite and removes fine particles from Ti ₂ O ₃ .
Helps generate AF. Furthermore, BN combined with N acts as a ferrite generation nucleus and refines the HAZ structure. In order to obtain this effect of B, the minimum
A B content of 0.0005% is required. However, if the amount of B is too large, coarse precipitates such as Fe ₂₃ (CB) ₆ will precipitate at the γ grain boundaries, deteriorating the low-temperature toughness. Therefore, it is necessary to limit the upper limit of the amount of B to 0.003%. Ca, REM controls the morphology of sulfide (MnS),
In addition to improving low-temperature toughness (increasing Charpy absorbed energy), it is also effective in improving hydrogen-induced cracking resistance. However, if Ca content is less than 0.001%, it has no practical effect, and if it exceeds 0.005%,
Large amounts of CaO and CaS are generated and become large inclusions.
It impairs not only the toughness but also the cleanliness of the steel, and also has an adverse effect on weldability. For this reason, the range of addition amount is
It was limited to 0.001-0.005%. REM also has the same effect as Ca, and its appropriate range is 0.001 to 0.005%. Now, the rolling method after reheating the cast slab in the present invention includes so-called normal rolling or processing heat treatment. Examples of processing heat treatment methods include (1) controlled rolling, (2) controlled rolling-accelerated cooling, and (3) direct rolling quenching-tempering. Most preferred is a combination of controlled rolling and accelerated cooling. Note that even if the product is reheated to a temperature below the Ac ₁ transformation point for the purpose of dehydrogenation or the like after production, the features of the present invention will not be impaired. Further, welding methods during pipe manufacturing include submerged arc welding, flash butt welding, electron beam welding, etc., and the features of the present invention will not be impaired even if welding is performed using any of the welding methods. (Example) In the converter-continuous casting-thick plate-tube manufacturing process, large-diameter steel pipes were manufactured from steel plates of various compositions, and the welded parts were
HAZ toughness was investigated. Note that the toughness of the HAZ part was measured using a Charpy test piece taken from a 1/2 t thick plate. Examples are shown in Table 1. The large diameter steel pipes manufactured by the method of the present invention all have good base metal properties and HAZ toughness, whereas the comparative steels not manufactured by the method of the present invention have poor HAZ toughness and are not suitable as steel plates used in harsh environments. Not. Among the comparative steels, Steel 8 and Steel 9 have too much Al content, so the number of Ti-containing oxides is small, the HAZ structure is not refined, and the HAZ toughness is poor. Steel 10, Steel 11, and Steel 12 have a low water density of secondary cooling water on the short side of the slab, so the solidification cooling rate is slow, and the number of Ti-containing oxides in the weld is small, so the HAZ The structure is not refined and the HAZ toughness is poor. Steel 13 cannot be used as a large-diameter steel pipe because the secondary cooling water on the short side of the slab has a high water density, which causes surface scratches on the slab.

【table】

[Table] * Unit: ppm

[Table] (Effects of the invention) The present invention makes it possible to inexpensively manufacture large-diameter steel pipes with excellent low-temperature toughness at welded parts, improving the safety of line pipes used in harsh environments such as extremely cold regions. , we were able to make a big improvement.

Claims (1)

[Claims] 1% by weight: C: 0.005 to 0.15%, Si: 0.5% or less, Mn: 0.5 to 2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.005% or less, Nb: 0.01~0.1%, Ti: 0.005~0.03%, N: 0.001~0.005%, O: 0.0015~0.006%, and the balance is iron and unavoidable impurities. During manufacturing, the secondary cooling water on the short side of the slab should be kept at a water density of 350~
840/ ^m2 , and 50 to 500 of Ti-containing oxide.
A method for producing a large diameter steel pipe with excellent low-temperature toughness, characterized by producing a slab containing 1/mm ² , followed by heating and rolling, and then butt welding the edges of the steel plates. 2% by weight C: 0.005-0.15%, Si: 0.5% or less, Mn: 0.5-2.2%, P: 0.02% or less, S: 0.005% or less, Al: 0.005% or less, Nb: 0.01-0.1%, Ti : 0.005~0.03%, N: 0.001~0.005%, O: 0.0015~0.006%, as required V: 0.01~0.1%, Ni: 0.05~1.0%, Cu: 0.05~1.0%, Cr: 0.05~ Contains one or more of the following: 0.5%, Mo: 0.05~0.4%, B: 0.0005~0.003%, Ca: 0.001~0.005%, REM: 0.01~0.05%, with the remainder consisting of iron and inevitable impurities. When producing slabs from molten steel by the continuous casting method, the secondary cooling water on the short side of the slab has a water density of 350 to 840/m ² and contains 50 to 500 Ti-containing oxides/mm ² . A method for producing large diameter steel pipes with excellent low-temperature toughness, characterized by producing slabs, followed by heating and rolling, and then butt welding the edges of the steel plates.